This invention relates to a propelling nozzle for a hypersonic engine and, more particularly to a propelling nozzle which includes a mushroom-shaped central body axially displaceable for changing the nozzle throat area. The outer wall of the propelling nozzle behind the nozzle throat area, transitions into increasedly widening sectional planes of an expanding circle.
A difficult requirement to meet for hypersonic engines that are appropriate for flying speeds between Mach 0 to Mach 7 is that a high conversion of exhaust gas energy into the gross thrust, or jet thrust, needs to take place at both low flying speeds below Mach 1 as well as at high speeds of Mach 7.
For this purpose, it has been suggested to provide combined turboramjet engines which, at low flying speeds, operate as gas turbojet engines with or without an afterburner and, above a certain flying speed, operate as ramjet engines, i.e., RAM-operation.
The nozzles of hypersonic engines are significantly different from conventional nozzle concepts. This is because of the much larger variation range of the decisive parameters. Particularly, the nozzle throat area must be varied at a ratio of 1:5. Further, the existing nozzle pressure ratio, which during operation rises from approximately 3 during take-off to a magnitude of 1,000 at hypersonic flight Mach number 7, thus in principle requires an enormously high variation range of the divergence.
The extremely high divergence is required at hypersonic flight Mach numbers because of the existing high nozzle pressure ratios, i.e., the ratio of the exhaust surface to the nozzle throat surface. The extremely high divergence cannot be implemented inside the nozzle. Therefore, in any case, an afterexpansion path is required which follows the nozzle and is created by the corresponding design of the airplane rear.
The known axially symmetrical convergent/divergent nozzles having a lamellar construction, as used, for example, in military afterburner engines, have a variation range of the nozzle throat area and the divergence which is much too small. Therefore, this type of nozzle cannot be used for the engines of the above-mentioned type.
In addition, convergent, axially symmetrical nozzles with axially displaceable central bodies are known where the throat surface can be adjusted within a wide range. So far, nozzles of this type have been used only in cases with three engines without any afterburning. This is because the cooling of the central body by air taken, for example, from the turbo-engine, presents problems.
Another nozzle construction known, for example, from German Patent Application P 39 12 330 is called a two-dimensional nozzle. Although this construction permits a large variation range of the throat area and of the divergence, the construction of such a nozzle is very expensive and results in a high weight.
All known concepts for axially symmetrical and two-dimensional nozzles that are taken into consideration for hypersonic application are each supplemented by afterexpansion paths which are formed by the airplane rear contour. The afterexpansion paths are used for supplementing the divergent nozzle section in such a manner that the expansion of the thrust jet, at least on the upper side, is guided through a fixed wall.
There is therefore needed a nozzle for a hypersonic engine of this type which, on the one hand, permits a high nozzle divergence, and, on the other hand however, also has sufficient inherent stability and low leakages. In addition, it should be possible to vectorize the jet upward or downward while maintaining the desired divergence by means of a targeted adjustment.
According to the invention, this need is met by a propelling nozzle having a variable geometry for a hypersonic engine. The propelling nozzle has a mushroom-shaped central body which can be displaced for changing the narrowest nozzle cross-section with respect to an outer wall. The outer wall widens at one point in the direction of the nozzle outlet in which two opposite areas of the widening outer wall are constructed as expansion flaps. In the area of the expansion flaps upward edges, the flaps can be pivoted transversely with respect to the engine axis about pivots parallel to one another.
The principal advantages of the present invention are that a substantial rotationally symmetrical structure and a thermal and mechanical stressing of the actual nozzle shroud exist in the forward nozzle part. This is particularly advantageous with respect to the stiffness.
The same applies to the displaceable mushroom-shaped central body. Because of the rotationally symmetrical contour, the thermal and mechanical stressing of the mushroom-shaped central body is also rotationally symmetrical and can therefore be absorbed in an advantageous manner.
A high variation range of the nozzle throat area is achieved by the axial displacement of the central body. The central body's adjusting mechanism is situated inside the structure cooled by cryogenic hydrogen.
Advantageously, leakages of the type that are unavoidable in the case of two-dimensional nozzles can be avoided up to the area of the upstream edges of the expansion flaps. However, at high supersonic flight Mach numbers or high internal pressures, this area is already in the supersonic range where, because of the preceding expansion or because of the low static pressures in the nozzle, the leakages are less in comparison to the subsonic range.
Another significant advantage of the present invention is that the upper expansion flap together with the lower expansion flap may be used for vectorizing the jet upward or downward while maintaining the desired divergence by means of a targeted adjustment. In this case, in the lower Mach number range in which the upper expansion flap is moved out anyhow, larger vectorization angles may advantageously be set by means of the adjusting of both flaps. In the upper Mach number range, smaller vectorization angles can be achieved by the adjustment of the lower expansion flap. The upper expansion flap remains in its upper end position.
It is also an advantage that the mechanical stressing of moments of the expansion flaps about their respective axis of rotation is low because of the pressure level which decreases in the hypersonic flow.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.